Refinement of benzene hexachloride



REFINENIENT OF BENZENE HEXACHLORIDE Application September 28, 1951,Serial No. 248,866

No Drawing.

6 Claims.

This invention relates to improvements in methods of treating crudebenzene hexachloride, and more particularly to a new and improvedcontinuous process for the isolation of a product containing an enhancedproportion of the gamma isomer.

Benzene hexachloride (1,2,3,4,5,6-hexachlorocyclohexane), as is wellknown, is prepared by the chlorination of benzene under conditions thatfavor addition, 'but not substitution, reactions. The product soobtained comprises a mixture of stereoisomers, of which five, designatedas alpha, beta, gamma, delta and epsilonthe alpha isomer being presentin preponderant amount-have been isolated and characterized. Of theseonly the gamma isomer, M. P. 113 C., possesses a significant amount ofinsecticidal activity. Pure gamma benzene hexachloride, known as lindaneis, therefore, an important article of commerce and processes for theisolation of products containing enhanced proportions of gamma benzenehexachloride have been the subject of recent intensive investigation.Several methods based upon relative solubilities of the isomers havebeen heretofore used for the isolation of such products. Since, in mostsolvents, the solubility of the gamma isomer is intermediate between thesolubrhties of the other isomers, extraction techniques, wherein thesolid mixture of benzene hexachloride isomers, hereafter designated ascrude benzene hexachloride, is contacted with a solvent having apreferential solubility for the gamma over the alpha isomer, inproportions such that only a part of the crude benzene hexachloride isdissolved, have been most extensively investigated. Among the solventsused for this purpose are included lower aliphatic alcohols as describedin U. S. Patent 2,553,956.

Thus, according to the teaching of the above patent, crude benzenehexachloride is treated with a solvent, e. g. methanol, in proportionssuch that substantially all the gamma benzene hexachloride, but not allthe alpha and beta benzene hexachloride, is dissolved. The solutionthereby formed is saturated with respect to the alpha and beta isomers,but contains less than the saturation amounts of the gamma, delta andepsilon isomers, together with small amounts of by-products, such asheptachlorocyclohexanes, which are formed in the chlorination reaction.The extract solution is separated from the undissolved solids, and ahigh-gamma fraction isolated and recovered by fractionalcrystallization, following which the highgamma fraction is separatedfrom its mother liquor by conventional solid-liquid separationtechniques.

Use of the foregoing procedure, it is true, results in isolation of aproduct contained an enhanced proportion of the gamma isomer. However,the yield of the highfraction, and the gamma content thereof, while atreasonably high levels, fall far short of those theoretically possible.

An important object of our 1nvent1on, therefore, 1s to provide a methodfor the refinement of benzene hexachloride in which a very high yield ofa product extremely rich in gamma benzene hexachloride is obtained. An-

2,760,995 Patented Aug. 28, 1956 other object is to provide an efiicientcontinuous process for the concentration of gamma benzene hexachloride,characterized by a high degree of recovery of gamma isomer. Stillanother object is to provide a method for gamma concentration in whichgamma isomer otherwise lost in a discard stream is recovered. Furtherobjects will be apparent from the following description.

We have made the surprising discovery that when a portion, but not all,of the mother liquor from the crystallization step, as described above,is recycled to the partial dissolution or extraction step, as describedabove, we obtain in our crystallized product an enhancement in gammacontent, and particularly a marked enhancement in recovery of gammaisomer. The copending application Serial No. 353,081 of GeorgeCalingaert (a division of his application Serial No. 135,022), nowabandoned, discloses recycling of all this mother liquor and claimsrecycling thereof generally. Our present invention is concerned withrecycling 2. portiononly of such mother liquor.

When our partial recycle is employed the proportion of gamma benzenehexachloride in the extract solution is lower than when either norecycle or complete recycle is used. In spite of this, yields and purityof the high gamma fraction obtained with partial recycle are markedlyhigher than those obtained with either no recycle or complete recycle.

The benefits of our invention are graphically illustrated in thefollowing three examples in which results we obtain our contrasted withresults obtained without recycle and with complete recycle of thecrystallization filtrate. Example i shows results when our process isemployed. All parts and percentages mentioned are on the weight basis.

Example I To an extraction vessel equipped with a mechanical stirrer,crude benzene hexachloride containing 12.2 per cent of gamma benzenehexachloride was fed continuously at a rate of 100 parts per hour.Simultaneously, methanol was fed to the extraction vessel at the rate of300 parts per hour. Vigorous agitation was maintained, and thetemperature was held by external cooling at 15 C. The extraction vesselwas sized so that the average residence time of the mixture therein wasapproximately two hours. Under these conditions the two-hour period wassufiicient to achieve equilibrium distribution of isomers between theliquid and solid phases, as determined by infrared analyses of samplesof the solution at varying time intervals from the time of initialcontacting of benzene hexa chloride and methanol. A slurry ofundissolved benzene hexachloride and methanol solution was continuouslywithdrawn from the extraction vessel and sent to one of two centrifugesmounted in parallel. In the centrifuge, separation of the liquid phasefrom the solid phase was accomplished. The centrifugate was sent to avaporizer and the solid cake removed from the centrifuge. During theremoval of the cake the slurry was sent to the alternate centrifuge tomaintain continuous operation. After distillation of 60 per cent of themethanol from the solution, the residue was transferred to acrystallizer and cooled therein so that the temperature after two hourswas 15 C. The slurry of crystals and mother liquor produced in thecrystallizer was sent to one of a second pair of centrifuges operatingin tandem. The solid cake on this centrifuge comprised the high-gammafraction. The centrifugate from this second centrifugation was splitinto two portions, one comprising about per cent of the centrifugate andthe other about 25 per cent. The larger portion was continuouslyrecycled to the extraction vessel, and the smaller portion discarded.When equilibrium had been reached, as evidenced by constancy or isomerdistribution in all liquid streams, determined by inethanol, n-propanol,isopropanol, n-butanol, sec-butanol, tert-butanol, isobutanol, equallybeneficial results are obtained.

The following two examples, in contrast, demonstrate '7 the results weobtainwith no recycle of the mother liquor,

and results obtained when -we recycle all the mother liquor,respectively.

Example .11

Crude benzene hexachloride containing 12.5 per cent of gamma'benzenehexachloride was processed in a manner identical with that in Example I,except that the centrifugate from the second centrifugation wascompletely discarded, and was'not recycled to the extraction vessel. Bythis -procedure,-only '37 percent of the gamma benzene'hexachloridepresent'in the crude charge was recovered, and the gamma content of thehigh-gamma fraction was only 73 per cent.

Example III Crude benzene hexachloride containing 13.6 per cent gammabenzene hexachloride wasprocessed in a manner identical with that inExample I, except that the centrifugate from the second centrifugationwas com pletely recycled, and none was discarded. The only removal ofmethanol from the system was that accomplished in'the-vaporizer. aOnly49 per cent of the gamma benzene hexachloride. presentin thecrndebenzene hexachloride charge was recovered, and the gammacontent of thehigh-gamma fraction was only 76 per cent.

From the above examples it can readily be seen that use of our inventionresults notonly in enchancement of the gamma content of the high-gammaproduct but particularly in a marked enhancement of the yield of gammaisomer. In the ordinary processonly 37 per cent of ,the gammaisomer isrecovered whereas 63 per cent, or almost two-thirds,.of this valuableinsecticide is discarded and lostoutright. 'When our process is employed83 per cent, or more than twice as much, of the gamma isomer isrecovered in a marketable condition. Even with-complete recycleof thefiltrate more than half of the gammadsomer isdiscarded. 'On this basisitcan readily be .seen that our process-can lead to significantincreases in the capacity: of lindane'manufacturing equipment; that is,withthe same equipment wecan turn out more than twice as much lindane orhigh-gamma benzene hexachloride'as can those employing theold method.

As mentioned above,,in most solvents the. delta benzene hexachlorideisomer is more soluble than the alpha and gamma isomers. This is .truein the case of methanol. -When theconcentration of deltabenzene'hexachloride in methanol solution builds upimproved results interms of yieldand purity of the high-gamma fraction are obtained.However, if thedelta concentration continues to build up to too highconcentrations then yields and purity of the high-gamma fraction beginto fall off. The reason-for this is not completely understood and wecannot state quantitatively .these concentration limits of delta benzenehexachloride. This is because analytical procedures for determining theconcentration of any particular benzene hexachlorideisomer in solutionare complicated by thezpresenee of other isomers and other impuritieswhich are present .in our mixtures. We have found, however, that if v wediscard enough of the recycle strearmto make the :amountofdelta isomerleaving the systemequal to the amount entering thesystem,thus preventing.any buildup vabove the optimum range of delta con en a ion, w obtainzenchanc yields 10f high gam- .between 0-per. cent and aboutper cent.

ma material. In fact, we find that we obtain some benefit'as long as werecycle any portion of the filtrate so long as some of the filtrate isdiscarded. The amount discarded can be very small relative to the amountrecycled. For example, we obtain good results when only about 5 per centof our recycle stream is discarded. Generally speaking, we prefer todiscard 15 to 60 per cent of our recycle stream'because within thisrange, depending up the delta content of the original crude benzenehexachloride, upon the extraction and crystallization temperatures, andupon the ratio of methanol to benzene hexachloride in the extractionstep, build-up of deltaisomer in the extract solutionis avoided.

7 An important variable in our process is the ratio of alcohol to crudebenzene hexachloride in our extractor. In general, we can operate with arather wide range of ratios of alcohol to benzene hexachloride. Forexample, we can use methanol in amounts just insufiicient to completelydissolve all-the benzene'hexachloride present. On the other hand, we canuse ratios of methanol to-benzene 'hexachloride so low as to justpermitliquid-solid separation to be made.

The preferred range of ratios of alcohol to benzene hexachloride dependson the temperature of extraction. In our preferred extractiontemperature range, as discussed below, we prefer to'use ratios ofmethanol to freshly'fed benzenehex-achloride of about 2.6/1 to about5.0/1 and in addition we-prefer that the ratio of methanol-toall-benzene hexachloride present-in the extraction step be approximately=1.5/1 to 2.5/1. Similar ratios are preferred when other alcohols areemployed.

The benefits-of ourinvention can berealized with-extractiontemperaturesranging from about 0 C. to as high as the refiux temperature ofthesolution employed.

'Howevenfor maximum yields and recoveries as well as the most economicoperation-weprefer to extract at temperatures ranging from about 10 C.to about 35 C. It

is-w-ithin thispreferred temperatureregionthatwe utilize our preferedratios of methanolto benzene hexachloride anol with about 3 per centwater. Under certain operating-conditions-we-obtain increased benefitswhen use of-this solventm-ixture is combined with our partial recycle.

The benefits of ourinvention are obtained when weremove by evaporation aportion of the solvent varying However, for best -yields and-recoveriesWe prefer to evaporate from about 30'per cent to'about 80 per cent ofthe-methanol prior to crystallization.

-Wecrystallize our highgammafraction from this concentrated solution attemperatures ranging from about 20C. to about60'C. We prefer tocrystallize in the temperature range lying between about 0 C. and 40 C.since this temperature-range enables us toobtain our best yields andpurities of'thehigh-gamrna fraction and in addition-permitsthe mosteconomical modeof operation.

From the foregoing, we have found that the amount of the-recycle streamto be discarded can be expressed as :a-function of thespe'cificgravityof the recycle solution according :to the following equation:

where -.y equals parts of recycle solution discarded per unittimeperpartof fresh benzene hexachloride charged perunitrtime and X equals thespecific gravity of the recycle solution. .Useof this equation permitsus to adjustourrecycle ratio to counteract natural fluctuations in ouroperation and permits us to discard the recycle .streamin proportionssuch that the delta isomer is discardedrasvfastastit isbeing introducedintothe system.

The followingexample illustrates .thedegree of control and the purity:and vyield of product we obtain by use of our preferred conditionstogether with the above equation.

Example IV Crude benzene hexachloride containing 13.1 per cent of gammabenezene hexachloride was processed in a manner identical with that ofExample I, except that the specific gravity of the centrifugate from thesecond centrifuge was continuously determined by a conventionalhydrometer, and the amount of centrifugate recycled to the extractionvessel continuously adjusted according to the equation:

where y equals parts of recycle solution discarded per unit time perpart of fresh crude benzene hexachloride charged per unit time, and Xequals the specific gravity of the recycle solution. During thisoperation the spe' cific gravity of the recycle solution varied betweenthe limit of about 0.990 and 1.080. However, by continually adjustingthe recycle flow according to the above equation, operation was carriedout smoothly, with optimum results. Sixty-seven per cent of the gammabenzene hexachloride charged in the feed was recovered as a 90 per centconcentrate.

We employ any of the customary means of solid-liquid separation, such asfiltration, centrifugation, decantation, etc. In general, centrifugationis preferred, giving rise to the most clean-cut separation of liquidsfrom solids.

We claim:

1. In a process for the production of a product containing an enhancedproportion of gamma benzene hexachloride by partialy dissolving crudebenzene hexachloride in a lower aliphatic alcohol, separating thetherebyformed solution from the undissolved solids, and crystallizing aproduct containing an enhanced proportion of gamma benzene hexachloridefrom said solution, the improvement comprising recycling a portion ofthe mother liquor from said crystallization step to said partialdissolution step, the portion recycled having essentially the samecomposition as the Whole quantity of the mother liquor.

2. The process of claim 1 in which the lower aliphatic alcohol ismethanol.

3. In a process for the production of a product containing an enhancedproportion of gamma benzene hexachloride by partially dissolving crudebenzene hexachloride in a lower aliphatic alcohol, separating thetherebyformed solution from the undissolved solids, and crystallizing aproduct containing an enhanced proportion of the gamma isomer from saidsolution, the improvement comprising recycling from about 40 per cent toabout per cent of the mother liquor from said crystallization step tosaid partial dissolution step, the portion recycled having essentiallythe same composition as the whole quantity of the mother liquor.

4. In a process for the production of a product containing an enhancedproportion of gamma benzene hexachloride by partially dissolving crudebenzene hexachloride in a lower aliphatic alcohol, separating thetherebyformed solution from the undissolved solids, and crystallizing aproduct containing an enhanced proportion of gamma benzene hexachloridefrom said solution, the improvement comprising recycling a portion ofthe mother liquor from said crystallization step to said partialdissolution step, the portion of mother liquor to be recycled beingdetermined by the equation:

where y equals parts of mother liquor discarded per unit time per partof crude benzene hexachloride fed per unit time, and X equals thespecific gravity of the mother liquor, the portion recycled havingessentially the same composition as the whole quantity of the motherliquor.

5. In a process for the production of a product containing an enhancedproportion of gamma benzene hexachloride by partially dissolving crudebenzene hexachloride in an aliphatic alcohol having from 1 to 4 carbonatoms, separating the thereby-formed solution from the undissolvedsolids, and crystallizing a product containing an enhanced proportion ofgamma benzene hexachloride from said solution, the improvementcomprising recycling a portion of the mother liquor from saidcrystallization step to said partial dissolution step, the portionrecycled having the same composition as the whole quantity of the motherliquor.

6. The process of claim 1 wherein up to percent of the mother liquor isrecycled.

References Cited in the file of this patent UNITED STATES PATENTS2,486,688 Thomas Nov. 1, 1949 2,585,898 Kauer Feb. 12, 1952 FOREIGNPATENTS 491,132 Belgium Mar. 16, 1950

1. IN A PROCESS FOR THE PRODUCTION OF A PRODUCT CONTAINING AN ENHANCEDPORPORTION OF GAMMA BENZENE HEXACHLORIDE BY PARTIALY DISSOLVING CRUDEBENZENE HEXACHLORIDE IN A LOWER ALIPHATIC ALOCHOL, SEPARATING THETHEREBYFORMED SOLUTION FROM THE UNDISSOLVED SOLIDS, AND CRYSTALLIZING APRODUCT CONTAINING AN ENHANCED PROPORTION OF PROVEMENT COMPRISINGRECYCLING A PORTION OF THE MOTHER PROVEMENT COMPRISING RECYCLING APORTION OF THE MOTHER LIQUOR FROM SAID CRYSTALLIZATION STEP TO SAIDPARTIAL DISSOLUTION STEP, THE PORTION RECYCLED HAVING ESSENTIALLY THESAME COMPOSITION AS THE WHOLE QUANTITY OF THE MOTHER LIQUOR.